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Surface emitter for web edge control

​Ultrasonic transducers are frequently used to monitor the web edge in the production of continuous materials as they provide numerous advantages. A transducer from SECO often serves as the sensor head.
The sensors detect the web edge with an accuracy of less than a millimetre. Regulating mechanisms ensure that the material is rolled up evenly.
When round single transducers are used, as illustrated in Figure 1, the limited linearity range of the transducer is sometimes seen as a disadvantage.

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Figure 1:  Round SECO ultrasonic transducer SC125.1

An ideal linear reduction of the reception signal in the case of increasing coverage of the transducer is not possible due to the round shape of the transducer.

Graph 1:  Reception signal in relation to the coverage of the emitter surface of a SECO single transducer SC125.1

​Graph 1 shows the relatively small linearity range of 3 to 4mm of a round single transducer SC125.1. Thus, in a yoke application, approximately 28% of the transducer diameter can be used for regulating.

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Figure 2: Laboratory sample of a polypropylene film transducer made of electret

​Theoretically, a rectangular film transducer should represent an ideal surface emitter which can be expected to have a particularly wide linearity range.
In collaboration with theInstitute of Sensor and Actuator Technology at the University of Applied Sciences in Coburg (ISAT), the deflection characteristics of such an electret film transducer were examined with a laser vibrometer.

Messung der Auslenkamplitude eines Polypropylen-Wandlers mit einem Laservibrometer (pp_folienwandler_messungen_isat.jpg)

Figure 3: Deflection and false-colour image of the laser vibrometer, measured at the Institute of Sensor and Actuator Technology (ISAT)

​The false-colour image in Figure 3 shows the emitter surface at the moment of the maximum amplitude. As almost the entire surface is coded in the same colour and thus the whole emitter surface of the film transducer vibrates with the same amplitude, the film transducer almost perfectly demonstrates the characteristics of an ideal surface emitter.

Graph 2: The reception signal of a film transducer in relation to the coverage of the emitter surface.

​Graph 2 shows the result of a shadowing test in a yoke application. Here, as in the case of the single transducer SC125.1, two polypropylene film transducers were tested in emitter - receiver operation.
One prong of the polypropylene film transducer shows linear signal attenuation across a range of 60mm. Thus over 85% of the effective transducer length can be utilised in a yoke application.
Unfortunately, polypropylene film transducers are very sensitive and only have limited applications in the industrial sector.
A comparable linearity range can also be achieved with conventional ultrasonic transducers from SECO if several transducers are arranged in an array.

Graph 3: The reception signal of a SC125.1 transducer array in relation to the coverage of the emitter surface.

​A set up of this kind also demonstrates a large linearity range (100mm in this example). Here too, over 85% of the effective length can be used for a yoke application.

Graph 4: Comparison of the amplitude attenuation in relation to the shadowing of different transducers in a standardised display.

​Graph 4 summarises this test once again in a standardised way.
In addition to the large linearity range of an array, the application of tried-and-tested technology brings further advantages.
SECO transducers are sturdy and are long lasting even in tough conditions.
The design of a transducer array can be adapted to virtually all installation geometry.
Thus, a process in available which provides a wide linear control range with tried-and-tested technology.